Raymond J. Longbottom

High temperature oxidation behaviour of ferritic stainless steel SUS 430 in humid air

The high temperature oxidation behaviour of ferritic stainless steel SUS 430 was investigated over the temperature range from 1000 to 1150 °C in humid air containing 18% water vapour. Isothermal thermogravimetric analyses were performed to study the oxidation kinetics. The microstructure, composition and thickness of the oxide scale formed were investigated via optical microscopy (OM), X-ray diffraction and a scanning electron microscope equipped with an energy dispersive spectrometer. The results indicate that breakaway oxidation occurs at all temperatures and that its onset is accelerated by increasing temperature. The growth rate of the multilayer oxide scale follows a parabolic law with apparent activation energy of 240.69 kJ/mol, and the formation of FeO is decreased when the temperature is higher than 1120°C. The inner oxide scale, Fe-Cr spinel, grows mainly inward and internal oxidation is observed even in a short oxidation test at 1150°C for 105 s. The mechanism of high temperature oxidation of SUS 430 in humid air containing 18% water vapour is discussed.

Strength and bonding in reduced ironsand–coal compacts

Abstract In this investigation, the strength and bonding within reduced ironsand–coal compacts were studied, with the aim of better understanding the binding mechanisms in the reduced compacts and, based on this understanding, to improve their strength. Ironsand ore and sub-bituminous coal were mixed and pressed into compacts, which were reduced by heating in a thermogravimetric furnace to temperatures between 1273 and 1573 K under argon. The progress of the reaction was monitored by measuring the weight loss with time. The reduced compacts were found to have low strength in compression testing. The main form of bonding between the reduced ironsand particles in the compact was by the formation of a slag-like material. Increasing the final reduction temperature was found to have a profound effect on the strength of the compacts by promoting the formation of this slag-like material.

Morphology and composition changes of spinel (MgAl2O4) inclusions in steel

Abstract In this study, spinel inclusions of close to stoichiometric MgO.Al2O3 composition and known size distribution were added to a liquid steel bath before assessing their reactivity. The inclusions were then tracked for changes in size, morphology and composition with time in an aluminium killed steel at 1600°C, by sampling of the melt followed by automatic SEM based inclusion analysis techniques. The majority of the inclusions in the melt were alumina and complex sulphide inclusions, with the added spinel inclusions being a small proportion of the total inclusions. The proportion of alumina inclusions increased with time. The spinel inclusions were found to be evenly distributed through the melt and there was little change in their average size during the reaction. However, the composition of the inclusions did change, with the Mg/Al mass-% ratio changing from ∼0·5 to 0·08.

A review of phosphorus partition relations for use in basic oxygen steelmaking

Phosphorus removal in basic oxygen steelmaking is a significant problem for integrated steelmakers. Phosphorus removal is required due to its deleterious effect on the mechanical properties of steel. However, this is progressively becoming more difficult due to the increasing phosphorus content of many iron ores. Many studies have investigated dephosphorisation and published empirical phosphorus partition (LP) equations for a range of conditions. The structure of these equations has been used to develop a new partition relation that allows the effect of minor slag constituents such as TiO2, Al2O3 and V2O5 on steel dephosphorisation to be tested. Al2O3 was found to have a weak negative effect on the measured LP, except at the lower oxygen potential range tested, where a positive correlation was observed. Increasing TiO2 and V2O5 contents were found to decrease the measured LP; however, these correlations became less prevalent at the higher oxygen potential ranges tested.

In situ neutron diffraction study of the reduction of New Zealand ironsands in dilute hydrogen mixtures

ABSTRACT The reduction of New Zealand titanomagnetite ironsand in a dilute hydrogen–nitrogen gas mixture was studied in situ using neutron diffraction. Neutron diffraction allowed in situ observation of large samples during reduction at high temperatures. Australian hematite ore, studied as a comparison, reduced much more quickly than the pre-oxidised ironsand, which in turn reduced more quickly than raw ironsand. The ironsand was predominantly titanomagnetite with small amounts of titanohematite. The rate of wüstite formation increased and metallic iron was formed only after the reduction of titanohematite. Experimental results confirmed the expected reduction pathway for initial reduction of titanomagnetite ore was described well by a three-interface shrinking core model. The rate controlling step in the reduction reactions studied was the mass transport of water vapour in the bulk gas. At higher temperatures, slow removal of water vapour meant that the pH2O increased, thus preventing reduction of wüstite to metallic iron.

The effect of combinations of iron-bearing minerals and quartz on coke reactivity

ABSTRACT The effect of combinations of iron-bearing minerals and quartz on the reactivity of coke analogues with CO2 was examined in a TGA. Troilite–quartz, pyrite–quartz and magnetite–troilite binary combinations and the troilite–magnetite–quartz ternary combination were examined. For the troilite–quartz and pyrite–quartz binary combinations the reactivity in general decreased non linearly with increasing Si:Fe ratio. The magnetite–troilite combination had a non-linear effect on the reactivity in that mixes of these two minerals show a higher reactivity than would be expected from a simple proportional mixing approach. Some evidence of sulphur transfer from the troilite to the magnetite was observed. The reactivity of the ternary magnetite–troilite–quartz combination can largely be understood from the trends in the axes. These were largely followed across all compositions, allowing better understanding and prediction of the effect of these minerals on the reactivity of industrial cokes.

Thermodynamic considerations of the corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Abstract Thermodynamic analysis was carried out to interpret the results of corrosion testing of nickel ferrite samples in cryolite-based baths. The equilibrium between cryolite-based baths and nickel ferrite was considered. Isopleths between cryolite-based baths and nickel ferrite confirmed that for the temperature range of interest (1223–1273 K) there was limited solubility of nickel ferrite in the bath. To better understand the formation of the metal from nickel ferrite, the effect of reducing potentials on nickel ferrite and nickel ferrite–cryolite-based bath systems were considered. The formation of a metal phase was predicted at relatively high pO2. The metal phase was nickel-rich at higher pO2, becoming enriched in iron as the pO2 decreased. The oxide phases seen in corroded nickel ferrite samples corresponded to the spinel phase in the thermodynamic calculations. Penetration of aluminium oxides into the spinel phase seen in the experimental samples occurred only under a reducing potential.

Reactivity of selected oxide inclusions with CaO-Al2O3-SiO2-(MgO) slags

Inclusion content minimization is key in the production high quality steels. This is typically achieved through approaches ranging from preventing inclusion formation via process control to optimising process conditions to remove any inclusions that form. Inclusion removal from steel is generally through reaction with a slag. The inclusion transfers across the steel-slag interface to dissolve in the slag. This transfer process is primarily a dynamic interfacial tension/wetting driven process. In this study, the dynamic wetting (θ) of a range of slags in the CaO-Al2O3-SiO2-(MgO) system on alumina (Al2O3), magnesia spinel (MgAl2O4) and calcium aluminate (CaO.Al2O3) substrates has been assessed using the sessile drop technique. The reactivity of selected inclusion phases was studied by evaluating the wetting results and characterizing the microstructure of the slag-inclusion interface. It was found the dependence of θ value on the composition was lower for the basic ladle type slags than the acid tundish type slags.